Fatty acid esters of short-chain aliphatic alcohols, particularly those containing up to 4 carbon atoms, and above all, fatty acid methyl esters have acquired considerable commercial significance. For example, they are important starting materials for the production of fatty alcohols, and are also used for the production of other oleochemical products, for example soaps, tensides, alkanolamides, etc.
On an industrial scale, fatty acid esters of lower alcohols are mainly produced by alcoholysis of the corresponding fats and/or oils of natural origin which, as already known, are fatty acid triglycerides. However, vegetable and/or animal fats and oils almost always contain considerable quantities of free fatty acids, this content of free acids being variable over a wide range, depending on the origin of the material and its previous history. The content of free fatty acids is almost always above 3% by weight. The acid number of the commercially available, crude coconut oil is normally not above 10-20. The acid number of other vegetable oils, particularly those of good quality, is below 10, poorer qualities having acid numbers of, for example, from 20 to 25. Commercial-grade tallows, which are valued and handled according to their acid number, generally have free fatty acid contents, depending on their quality, of from 1 to 15-20% by weight, corresponding to an acid number of from about 30 to 40 and, in some cases, even higher.
The acid number of the triglyceride used for transesterification has a very considerable bearing upon the possibilities and process conditions of the transesterification reaction.
Accordingly, the production of fatty acid esters on an industrial scale by the alcoholysis of fats and/or oils may be carried out by various methods:
In the presence of alkali catalysts, neutral fats may be smoothly converted into the corresponding alkyl esters with a 50 to 100% excess over and above the stoichiometrically necessary quantity of alcohol at temperatures as low as 30.degree. to 70.degree. C. In this case, however, it is only possible smoothly to react fats and oils of which the free fatty acid content is below 0.5% by weight, corresponding to an acid number of the triglycerides of approximately 1 and lower.
The Bradshaw process used in industry is based, for example, on the alkali-catalyzed transesterification of fats, of which the acid number should not be above 1.5, with methyl alcohol as the first stage of a continuous soap manufacturing process, cf. for example Ullmann, Enzyklopadie der technischen Chemie. 3rd Edition, Vol. 7, pages 525 et seq; 4th Edition, Vol. 11, pages 490 et seq.
By means of another industrial process (cf. Ullmann loc. cit., 4th Edition, Vol. 11, page 432), it is even possible to transesterify fats and oils having higher acid numbers. In this process, however, the production of fatty acid methyl esters is carried out at 240.degree. C. under elevated pressure (approximately 100 bars) in the presence of alkali or zinc catalysts with a large excess of methanol (7 to 8-fold molar excess).
On account of the almost always considerable content of free fatty acids in commercial fats and oils of natural origin, pressureless transesterification (which is advantageous in terms of energy by virtue of the lower temperatures involved and the distinctly lower methanol demand and which does not require the use of pressure reactors) presupposes a reduction in the acid number, for example by preliminary conversion of the free fatty acids into the corresponding alkyl or glycerol esters.
According to Ullmann, loc. cit. 4th Edition, Vol. 11, page 432, this preliminary esterification reaction may be carried out at 240.degree. C.20 bar in the presence of alkali catalysts. In this case, too, expensive pressure reactors have to be used for the preliminary esterification with methanol and other short-chain alcohols.